Introduction

The C-Band All Sky Survey (C-BASS) is a project to
image the whole sky at a wavelength of six centimetres (a frequency
of 5 GHz), measuring both the brightness and the polarization of the
sky.

C-BASS employs very sensitive microwave amplifiers,
cooled to within a few degrees of absolute zero, and configured to
measure tiny differences in temperature and polarization. They are
mounted on two separate telescopes — one at the Owens Valley
Observatory (OVRO) in California, the other in South Africa. This
allows C-BASS to observe both in the northern and southern
hemispheres and hence map the whole sky.

C-BASS is a collaborative project between the
Universities of Oxford and Manchester in the UK, the California
Institute of Technology (supported by the National Science
Foundation) in the USA, the Hartebeesthoek Radio Astronomy
Observatory (supported by the Square Kilometre Array project) in
South Africa, and the King Abdulaziz City for Science and Technology
(KACST) in Saudi Arabia. The southern telescope is a 7.6-m dish
donated to the project by Telkom. The northern telescope is a 6.1-m
dish donated by the Jet Propulsion Laboratory.

Status (June 2014)

The northern instrument is conducting survey observations at
OVRO. We are working on final calibration of intensity and polarization. The southern instrument has completed its commissioning phase at
Hartebeesthoek Radio Astronomy Observatory and has been moved to its observing site at Klerefontein near Carnarvon in the Karoo (Northern Cape Province), and is conducting the southern survey observations.

Goals

The main uses of this survey will be to help us make better
images of the cosmic microwave background (CMB) and to study diffuse
radiation from our Galaxy. The CMB is the very faint afterglow of
the Big Bang. It has a temperature of less than three degrees
Celsius above absolute zero. By making images of this radiation,
scientists are able to see the universe as it was just after the Big
Bang. This radiation is also polarized — in a similar way to
sunlight reflected off water — and this can be measured in a
way similar to wearing polarized sunglasses. The polarization is
predicted to have two distinct patterns; one due to variations in
density, the other due to the presence of gravitational waves. The
polarization patterns from gravitational waves are expected to be
extremely faint (less than 1 millionth of a degree Celsius) but if
they can be measured, they will provide information about the state
of the universe when it was less than a billion billion billionth of
a second old.

Many telescopes are now being designed and built to detect these
very faint polarization patterns in the CMB but, in order to clearly
see the signals, we need to accurately remove all the other
contaminating signals from the sky which obscure our view of the
early universe. Most of this contamination comes from our own
Galaxy, the Milky Way. This is where C-BASS will play an essential
role in this quest for understanding the origin and evolution of our
universe. Operating at a wavelength of 6cm, C-BASS will make an
extremely accurate map of the contaminating signal from our
Galaxy. This will allow the contamination to be subtracted with
great accuracy from high-frequency measurements such as those being
made by Planck.

The all-sky maps produced by this survey will be key to enabling
accurate subtraction of contaminating signals from the data
collected by specialized CMB telescopes in order that they can
reveal the true fluctuations in the microwave background. In
addition to playing an essential part in the quest for revealing the
tiny fluctuations in the CMB and understanding the origin and
evolution of our universe, C-BASS will vastly increase our
understanding of the physics of the gas between the stars in our own
Galaxy, for example by mapping out the magnetic field in the
Galaxy.

The Northern C-BASS 6.1m dish, at the Owens Valley Radio
Observatory, California. The secondary mirror is located on a foam
cone support. The large shielding around the primary mirror improves
sidelobe performance.

The Southern C-BASS 7.6m dish at Klerefontein, near Carnarvon, South Africa (June 2014). This telescope has a similar foam cone to support the secondary mirror, but the optical configuration is Cassegrain rather than Gregorian.